What it measures
Low point is the distance along the target line from the clubhead's center at maximum compression to the lowest point of the swing arc. It is given in inches or centimeters, along with whether the bottom came before or after the ball. It is calculated from the club's motion, not from where a divot or mat mark appears.
When the bottom of the arc is after the ball, on the target side, the club was still moving down at impact, so the attack angle is negative. When the bottom is before the ball, the club was already rising, so the attack angle is positive. It describes the same strike as attack angle, but in a form that is easier to picture on the ground.
Typical numbers
Skilled iron players bottom out after the ball. Good players with a 6 iron typically bottom out around 4 to 5 inches past the ball, and a tour 7 iron typically bottoms out roughly 3 to 5 inches past the ball. Wedges are similar or a little further forward.
With the driver, players who hit up have a low point a few inches before the ball. Everyday golfers vary far more from swing to swing. Some bottom out at or behind the ball, which produces fat and thin shots, while others with steep swings bottom out well past the ball and take deep divots. The numbers vary with swing length and setup, so treat consistency as more important than one exact figure.
What changes it
Ball position relative to where your swing bottoms out is the first factor. The same swing measured with the ball further back shows a low point further in front of the ball. Body pressure is the other big one: shifting weight and the chest toward the lead side moves the low point forward, hanging back moves it behind the ball.
Attack angle and low point move together. As a rough rule for a full iron swing, each extra degree of downward attack angle moves the bottom of the arc about an inch further past the ball. Early release of the wrists, a collapsing lead arm, or reaching for the ball tend to move it back.
What it changes
Low point decides strike quality more than almost anything else. Bottoming out a few inches after the ball gives ball first contact with irons, steady spin and predictable distance. Bottoming out at or behind the ball leads to fat strikes, thin strikes, lost ball speed and high, inconsistent spin.
Through attack angle, it also shapes spin loft, launch and spin. A low point well past the ball means a steep strike, more spin and lower launch; a low point behind the ball with a driver means hitting up, less spin and higher launch.
How to use it
Mats in a simulator forgive fat shots because the club bounces off the surface instead of digging, so low point is your honest feedback about contact. With irons, aim for a consistent low point a few inches past the ball. With the driver, a low point behind the ball usually means you are hitting up.
Bay drill: place a towel or headcover about 4 inches behind the ball (well clear of the club path) and hit ten 7 irons without touching it, watching low point after each shot. Then try a pause at the top of the swing with a deliberate shift into the lead foot before swinging down, and see whether low point moves forward.